5:15 PM - 6:45 PM
[SVC29-P01] Monitoring method of river water quality using the field auto-sampling tool at Hakone volcano
Keywords:Hakone volcano, Owakudani, monitoring of water quality, automated sampling, Cl-/SO42- ratio
Since 2001, volcanic unrests with earthquake swarms and inflation of the edifice have been observed every few years in Hakone Volcano, and very small phreatic explosion occurred at Owakudani in 2015. The monitoring of the water quality of the Owakuzawa River, which flows through Owakudani, revealed that the concentration the Cl-/SO42- ratio increased immediately after the 2015 eruption (Kikugawa et al., 2018). This phenomenon may reflect an increase in the supply of magma-derived volcanic gases such as hydrogen chloride. However, because the frequency of monitoring was about once a month, detailed changes in water quality before and after the eruption could not been detected.
On the other hand, the water quality of the Owakuzawa during calm periods may change due to rainfall and anthropogenic factors (e.g., maintenance of hot spring facilities using steam). Understanding the details of these changes will provide basic data for evaluating the disappearance of volcanic activity through water quality, but conventional field monitoring still faces the problem of infrequent water sampling. In this study, we attempted to monitor water quality at a high temporal resolution by installing the field auto-sampling tool at the Owakuzawa, and examined short-term water quality fluctuations and their factors.
The field auto-sampling tool (GOFAT, Ono et al., 2020) was used to monitor water quality in the Owakuzawa. The field auto-sampling tool was installed downstream of a weir approximately 5 m high. A polyethylene tube (inner diameter: 15 mm) approximately 15 m long was used to provide a flow path for the river water to a storage container attached to the field auto-sampling tool. The water in the container is constantly replaced by draining off overflow water. The field auto-sampling tool pumps river water from the storage container and fills the vials at predetermined time intervals. Water temperature and pH were measured using loggers installed in the storage vessel. After the vials were collected, the concentration of the samples was determined using Ion Chromatography.
Water quality monitoring was conducted between October 13 and 17, 2023, using water sampling every 3 hours, and the Cl-/SO42-ratio increased from 0.22 to 0.26 between 0:00 and 12:00 on October 15. Since rainfall was recorded during the period, it is thought that the Cl-/SO42-ratio captured a temporary fluctuation due to rainfall. In addition, temporary decreases in pH with a peak around 10:00 daily was observed. Since the maintenance work at the hot spring development facility is conducted around the time, it is assumed that this water quality fluctuation is caused by human factors. Next, to understand the details of this water quality fluctuation, water samples were taken once every hour from 9:00 to 12:00 between November 10 and 16. Comparing each hour of the 7-day period, the Cl-/SO42-ratio was low at 9:00 and the daily fluctuation was small, whereas it increased at 10:00 and the daily fluctuation was large. At 11:00 and 12:00, the Cl-/SO42-ratio decreased again and the diurnal variation was small. The field auto-sampling tool can be used for monitoring water quality in the Owakuzawa. Furthermore, the characteristics of the Cl-/SO42- ratio during rainfall and periodically during the day are useful basic data for the assessment of volcanic activity.
On the other hand, the water quality of the Owakuzawa during calm periods may change due to rainfall and anthropogenic factors (e.g., maintenance of hot spring facilities using steam). Understanding the details of these changes will provide basic data for evaluating the disappearance of volcanic activity through water quality, but conventional field monitoring still faces the problem of infrequent water sampling. In this study, we attempted to monitor water quality at a high temporal resolution by installing the field auto-sampling tool at the Owakuzawa, and examined short-term water quality fluctuations and their factors.
The field auto-sampling tool (GOFAT, Ono et al., 2020) was used to monitor water quality in the Owakuzawa. The field auto-sampling tool was installed downstream of a weir approximately 5 m high. A polyethylene tube (inner diameter: 15 mm) approximately 15 m long was used to provide a flow path for the river water to a storage container attached to the field auto-sampling tool. The water in the container is constantly replaced by draining off overflow water. The field auto-sampling tool pumps river water from the storage container and fills the vials at predetermined time intervals. Water temperature and pH were measured using loggers installed in the storage vessel. After the vials were collected, the concentration of the samples was determined using Ion Chromatography.
Water quality monitoring was conducted between October 13 and 17, 2023, using water sampling every 3 hours, and the Cl-/SO42-ratio increased from 0.22 to 0.26 between 0:00 and 12:00 on October 15. Since rainfall was recorded during the period, it is thought that the Cl-/SO42-ratio captured a temporary fluctuation due to rainfall. In addition, temporary decreases in pH with a peak around 10:00 daily was observed. Since the maintenance work at the hot spring development facility is conducted around the time, it is assumed that this water quality fluctuation is caused by human factors. Next, to understand the details of this water quality fluctuation, water samples were taken once every hour from 9:00 to 12:00 between November 10 and 16. Comparing each hour of the 7-day period, the Cl-/SO42-ratio was low at 9:00 and the daily fluctuation was small, whereas it increased at 10:00 and the daily fluctuation was large. At 11:00 and 12:00, the Cl-/SO42-ratio decreased again and the diurnal variation was small. The field auto-sampling tool can be used for monitoring water quality in the Owakuzawa. Furthermore, the characteristics of the Cl-/SO42- ratio during rainfall and periodically during the day are useful basic data for the assessment of volcanic activity.